Ph.D. Princeton University. Materials Science and Engineering. 2001.
Ph.D. Tsinghua University. Solid Mechanics. 1998.
M.S. Tsinghua University. Solid Mechanics. 1995.
B.S. Tsinghua University. Engineering Mechanics. 1994.
Multi-scale and multi-physics modeling and experimental approaches to address emerging challenges in energy, nanomechanics and mechanobiology across multiple scales, such as self-assembled nanostructures and their applications in materials, mechanics, energy and biological systems, morphological evolution and properties of nano/micro scale structures, and mechanical electrochemical processes in Li-ion battery systems.
Experimental evidence has accumulated in the recent decade that nanoscale patterns can self-assemble on solid surfaces. A two-component monolayer grown on a solid surface may separate into distinct phases. Sometimes the phases select sizes about 10 nm, and order into an array of stripes, disks or other regular patterns. These behaviors are intriguing because they are absent in bulk phase separation. The ability of patterning nanostructures on a surface is very important for many modern technological applications, such as in microelectronics circuits and digital storage media. It also opens up the possibility of fabricating cheap, large area devices using non‑ lithographic techniques.
Why do atoms self-assemble? What sets the feature size? The answers differ for different material systems. A unifying concept, however, can be identified. For many reasons the free energy of a material system depends on its configuration (e.g., the composition of the phases and their spatial arrangement). When the configuration changes, the free energy also changes. This defines thermodynamic forces that drive the configuration change. The change is effected by mass transport processes, such as diffusion. To assemble a nanostructure, some of the forces must act over the scale comparable to the feature size, and are therefore much longer ranging than atomic bond length.
We developed a thermodynamic framework to study the remarkable phenomena. We developed the numerical technique and performed large-scale simulation of the whole process of formation and evolution of nanostructures. The studies reveal remarkably rich dynamics and suggests a significant degree of experimental control in growing ordered nanoscale structures. Our work reveals how various parameters, including free energy of mixing, phase boundary energy, surface stress, average concentration, anisotropy etc, may influence the nanostructures. The model and simulation technique provides a powerful tool to conduct “numerical experiments” and investigate various behaviors associated with nanostructure evolution. We are now studying guided self-assembly, i.e. how to grow desired fine nanoscale structures by pre-patterning some coarse structures.
Examples systems: self-assembled thin films, novel nanofabrication technique by exploiting electric field and interface instability, patterning nanostructures via molecular dipoles and double layer charges, self-assembly of nanoparticles and filaments, mechanism of capillary forming of 3D nanostructures
Converting a large portion of the transportation sector to electricity is critical to solve our energy and pollution problems. By integrating across disciplines, we develop critical understanding of the battery fading mechanisms across multiple length and time scales from SEI layer formation to mechanical stresses. The research work provides an integrated picture of the complicated battery fading process, which will be fed into a control algorithm for optimized battery cycle life and performance. For instance, our work showed how coupled phase transition and Li intercalation contributes to the stress evolution in electrode materials, and the subsequent crack formation and loss of active materials during charge/discharge. In another work we showed how dissolution in both active and inert material phases contributes to the degradation of composite Li-ion electrodes.
Z. Zhao and W. Lu, “Spontaneous propagation of self-assembly in a continuum medium,” Physical Review E, 85, 041124, 2012
J. Park and W. Lu, “Self-assembly of nanoparticles into heterogeneous structures with gradient material properties,” Physical Review E, 83, 031402, 2011
J. Park, W. Lu, and A.M. Sastry, “Numerical simulation of stress evolution in lithium manganese dioxide particles due to coupled phase transition and intercalation,” Journal of the Electrochemical Society, 158, A201-A206, 2011
M. De Volder, S. Tawfick, S.J. Park, D. Copic, Z. Zhao, W. Lu, and A.J. Hart, “Diverse 3D microarchitectures made by capillary forming of carbon nanotubes,” Advanced Materials, 22, 4384-4389, 2010
S. Lee and W. Lu, “Effect of Mechanical Load on the Shuttling Operation of Molecular Muscles,” Applied Physics Letters, 94, Art. No. 233114, 2009.
Q. Wei, J. Lian, W. Lu, and L. Wang, “Highly Ordered Ga Nanodroplets on a GaAs Surface Formed by a Focused Ion Beam,” Physical Review Letters, 100, Art. No. 076103, 2008.
D. Salac and W. Lu, “A Local Semi-Implicit Level-Set Method for Interface Motion,” Journal of Scientific Computing, 35, 330-349, 2008.